This invention relates to a lash adjuster for automatically adjusting a valve clearance present in a valve actuator mounted to an internal combustion engine.
A valve actuator includes cams which rotate about a camshaft to open and close intake and exhaust valves (hereinafter simply referred to as valves). A lash adjuster is mounted between a cam and a valve to automatically adjust a valve clearance.
Such lash adjusters are disclosed in unexamined JP patent publications 11-324617 and 11-324618. Either of the lash adjusters disclosed in these publications comprises a lifter body including an end plate having a top surface in contact with a cam and a bottom surface formed with a threaded blind hole, and an adjuster screw having its external thread in threaded engagement with the internal thread of the threaded hole. An elastic member is received in the threaded hole between its top wall and the adjuster screw to axially bias the adjuster screw. Each of the internal thread of the threaded hole and the external thread of the adjuster screw has a pressure flank adapted to be pressed against the pressure flank of the other of the internal thread and the external thread while the adjuster screw is being biased upwardly by the valve stem, thereby bearing a load applied to the adjuster screw, and a clearance flank facing opposite to the pressure flank and having a flank angle smaller than the pressure flank. As a whole, both the internal thread of the threaded hole and the external thread of the adjuster screw have a sawtooth-like axial section.
This lash adjuster is mounted between a cam and the stem of a valve. A valve spring mounted around the valve stem biases the valve stem toward the cam, thereby pressing its top end against the bottom end of the adjuster screw, and pressing the end plate of the lifter body against the cam. When the cam rotates, the lash adjuster, the valve stem and the valve are all pushed down against the force of the valve spring, thus opening the valve port and then pushed up by the valve spring until the valve port is closed by the valve.
With the lash adjuster mounted on an internal combustion engine, the distance between the top end of the valve stem and the camshaft while the valve is closed may increase due e.g. to thermal expansion of the cylinder head. If this happens, the adjuster screw will quickly move axially downwardly while turning with the clearance flank of its external thread sliding on the clearance flank of the internal thread of the threaded hole, thereby absorbing any gap (valve clearance) between the top end of the valve stem and the bottom end of the adjuster screw.
While the adjustor screw is being biased upwardly by the valve stem, the pressure flank of the external thread of the adjuster screw is pressed against the pressure flank of the internal thread of the threaded hole. In this state, since the pressure flanks have a larger flank angle than the clearance flanks, the adjuster screw cannot turn and thus cannot move axially relative to the lifter body.
The distance between the top end of the valve stem and the camshaft when the valve is closed may decrease when e.g. the valve seat becomes worn. If this happens, due to fluctuating loads applied to the adjuster screw from the valve stem, the adjuster screw will be gradually pushed into the threaded hole until the valve is completely seated on the valve seat when the cam is in contact with the end plate of the lifter body at its base-circle portion. This will prevent leakage of pressure even if the valve seat becomes worn. The adjuster screw is pushed into the threaded hole until the fluctuating loads disappear and then any gap present between the pressure flanks disappears.
In normal operating conditions, however, the adjuster screw scarcely turns relative to the lifter body. It only moves axially relative to the lifter body within the range determined by any axial gap between the internal thread of the threaded hole and the external thread of the adjuster screw.
To be more specific, the adjuster screw moves axially relative to the lifter body such that the pressure flanks of the internal and external threads repeatedly collide against and separate from each other.
In order for such a lash adjuster to operate stably and reliably, it is important to maintain the friction coefficients between the pressure flanks and between the clearance flanks within suitable ranges.
Specifically, the friction coefficient between the pressure flanks has to be high enough such that the pressure flanks cannot practically slide or slip relative to each other under normal operating conditions. If the friction coefficient between the pressure flanks is low to such an extent that the pressure flanks can slide relative to each other under normal operating conditions, every time the lifter body is pushed down by the cam, the adjuster screw will be pushed into the threaded hole while rotating with the pressure flanks sliding relative to each other. Since the adjuster screw is pushed into the threaded hole, the valve lift tends to be insufficient.
If the friction coefficient between the clearance flanks is higher than a certain level, the adjuster screw will be unable to rotate in such a direction as to protrude from the threaded hole even if the valve clearance increases due e.g. to thermal expansion of the cylinder head. If the valve clearance is left unabsorbed, the adjuster screw will collide hard against the valve stem, producing much noise.
The friction coefficients between the flanks of the threads change when and if:    (1) the viscosity of oil increases as the ambient temperature drops,    (2) the surface roughness of the flanks of the threads lowers due to long-term wear, and/or    (3) a low-friction oil containing such additives as molybdenum (Mo) is used.
In unexamined JP patent publication 3-501758 and U.S. Pat. No. 4,981,117, in order to prevent any change in the friction coefficient between the pressure flanks due to increased viscosity of oil at a low temperature, grooves are formed in the pressure flanks to divide the respective pressure flanks into a plurality of separate sections, thereby effectively expelling any oil present between the pressure flanks when the pressure flanks move close to each other.
In order to prevent any change in the friction coefficient due to lowered surface roughness as a result of long-term wear, unexamined JP patent publication 2003-193811 proposes to increase the surface roughness to a level greater than the expected depth of wear of the flank surfaces.
On the other hand, one effective way to prevent any change in the friction coefficient due to the use of a low-friction oil is to modify the substance forming the thread surfaces to a substance which is inert with respect to the low-friction oil, thereby suppressing reaction of the thread surfaces with the additives in the oil.
Typical substances that are inert with respect to a low-friction oil include hard films of ceramics and diamond-like carbon. Ordinarily, before forming such a film on one of the thread surfaces, the thread surface is finished to a low surface roughness so as to increase the bond strength between the film and the thread surface.
Such a hard film is formed on one of the thread surfaces to reduce wear of the mating thread surfaces and to reduce the friction coefficient therebetween.
Among the parts of a valve actuator, the cams and the slidable lifters or adjusting shims tend to become worn most severely by coming into sliding contact with each other. In unexamined JP patent publication 2003-13710, in order to reduce wear of these parts, oil-keeping dimples are formed in the outer peripheries of the cams, the surface of the lifter or adjusting shim that is brought into sliding contact with each cam is finished with a small surface roughness, and a hard film as mentioned above is provided on the thus finished surface.
Also, in many of conventional lash adjusters, one of the external thread formed on the adjuster screw and the internal thread of the threaded hole in which is received the adjuster screw has its surface formed with a rugged surface, the other of the threads has its surface finished with a small surface roughness, and a hard film is provided on the surface finished with a small surface roughness.
In this type of lash adjuster, i.e. the type that includes an adjuster screw, under normal operating conditions, the external thread and the internal thread scarcely slide relative to each other. They only repeatedly collide against and separate from each other. In this regard, a conventional hard film as described above serves no practical purpose.
Also, while it is required that at least the friction coefficient between the pressure flanks be sufficiently high, the last-mentioned conventional arrangement cannot fulfill this object, either. Specifically, when the internal and external threads become worn to a certain extent, the protrusions of the rugged surface will be polished by the hard film, while the hard film will be polished by the rugged surface. This results in a sharp reduction in the surface roughness of either of the pressure flanks, which in turn causes a sharp drop in the friction coefficient between the pressure flanks.
As described above, if the friction coefficient between the pressure flanks is insufficient, every time the lash adjuster is pushed down by the cam, the adjuster screw will be pushed into the threaded hole while turning with the pressure flanks of the internal and external threads sliding relative to each other. The valve lift decreases as a result.
Since the internal and external threads repeatedly collide against each other under normal operating conditions, a crack may develop in the hard film. A crack that has formed in the film, even a small one, tends to grow rapidly in a short period of time.
An object of the invention is to provide a lash adjuster of the above-described type which can keep the friction coefficients between the pressure flanks and between the clearance flanks in predetermined ranges.